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From the bubbly red fluid that fills the glass bottles adorned with foreign lettering, to the more familiar cans of soda such as Fanta and Coca-Cola that line the supermarket aisles, the number of carbonated beverages has increased steadily over the past century and they are widely available in many cultures today. Centuries ago, when Joseph Priestley discovered a method in which a gas could be dissolved into water, he hypothesized that these new carbonated drinks were capable of curing scurvy in sailors. Though this was soon shown to be untrue, 276 years later scientists in the University of California in San Diego have discovered the receptor cells in mammals that are responsible for tasting carbonation.

In order to understand the effects of carbon dioxide (CO2) – the primary dissolved gas found in carbonated drinks – on taste receptor cells (TRCs), a group of scientists lead by Dr. Charles S. Zuker monitored the electrophysiological responses that carbonation induced. When TRCs of mice were exposed to different concentrations of CO2, action potentials in one of the major nerves of the tongue demonstrated an increase in frequency of electrical responses as the concentration of CO2 increased. These electrical responses, known as action potentials, were shown to be dose-dependent and were caused both by the exposure to carbonated drinks, gaseous CO2, and CO2 dissolved in a buffer solution. As a control, the scientists exposed TRCs to pressurized air to demonstrate that action potentials were indeed caused by CO2 and not the sensation of the pressured air.

Once the scientists understood that CO2 in carbonated drinks causes a taste response, they were interested in identifying the type of TRCs responsible for tasting CO2. To test this, the scientists genetically ablated each type of taste cell separately. Among them one group of mice would lack sweet TRCs, another group would lack sour TRCs, and a third group would lack bitter TRCs. All groups of mice showed responses to CO2 exposure in gaseous and dissolved forms except for the group that lacked sour TRCs. These results suggest that the cells that sense sourness are responsible for sensing CO2 found in carbonated drinks.

Another question that the scientists sought to answer was which enzyme is responsible for breaking down CO2 into the stimuli that cause the taste sensation? It was previously known that Car4, an enzyme known as carbonic anhydrase, plays a role in breaking down CO2. Therefore, scientists knocked out Car4 in mice and observed whether CO2 detection was altered. This data would give the scientists an insight into the role that Car4 plays in the tasting of carbonation. Their results showed that the responses to CO2 were severely reduced when Car4 was not present, yet responses to other taste stimuli including those that are sour did not have any altered effects in the Car4 deficient mice.

Dr. Zuker and his team of scientists have demonstrated that sour taste receptor cells are responsible for the tasting of carbonation and that the presence of Car4 is required for these sour cells to taste the CO2. This research is an interesting breakthrough in science since it is applicable to both scientists interested in the biological mechanisms of sensing taste, and the non-science folk who learn about the science behind the consumption and tasting of such a common beverage like soda.